In verses 37-38, Jesus pronounces a blessing on those whom the Master finds watching when He returns. It is not that they have their noses pressed to the glass, watching for His return. Instead, those who are vigilant and careful in their responsibilities will be blessed. They are watching over the Master's house, ensuring that all is in order, even if it means sleepless nights. "Be ready" in verse 40 is a simple summation of the "watching" He desires.
Verse 38 warns that He might return in the second watch or in the third. Regardless of whether the Master returns early or late (from our perspective), He wants His servants to be ready and His household in order. He wants them to be maintaining the house, diligent in their duties, so that all is prepared for His return. If they spend their days staring out the window, watching the road for His return rather than fulfilling their duties, they will be neglecting what He has charged them to do.
The duties of a typical servant include many mundane, monotonous, and repetitive chores. It is easy for a servant to think, "What is the use? Do I really have to do this right now? Since there is no sign of the Master right now, perhaps I can just relax, and prepare quickly when His return seems near." Such a servant would be inclined to spend more time watching from the window for the Master's return than he would be performing his assigned tasks. Yet, a servant's responsibility is to be prepared and to make sure the household (the church) is prepared, not to anticipate the timing of the Master's return.
Jesus says repeatedly that we will not know. If we believe Him, our focus will be on being faithful and vigilant in the things He has given us to do. His return will take the household by surprise—there is no other way to understand His many statements. The critical point is the state of readiness and the usefulness of the household and the servants when He returns. If the household is not ready, or if the servants have been sleeping rather than working, they will face His wrath.